1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit that is composed of a thin film transistor (hereinafter referred to as TFT). For instance, the invention relates to an electro-optical device represented by a liquid crystal display device and to electric equipment having an electro-optical device as its component. Note that the term semiconductor device herein refers to a device in general which functions through utilization of semiconductor characteristics, and that the above electro-optical device and electric equipment fall into this category.
2. Description of the Related Art
In recent years, voluminous research has been made on a laser annealing technique for crystallizing an amorphous semiconductor film formed on an insulating substrate made of glass or the like, or for improving crystallinity of a partially crystallized semiconductor film. The amorphous semiconductor film is often formed of silicon.
Glass substrates are inexpensive and more processible compared with quartz substrates widely used in the past, and they can have a large surface area without causing any difficulty. These are the reasons for the recent active research mentioned above. A laser is preferred in crystallizing a film on a glass substrate because glass substrates in general have a low melting point. In laser annealing, only the amorphous semiconductor film on the glass substrate is given high energy without increasing the temperature of the substrate much.
A crystalline semiconductor is composed of a lot of crystal grains and hence is also called a polycrystalline semiconductor. A crystalline semiconductor film formed by laser annealing has high mobility. Therefore, a thin film transistor (TFT) formed from this crystalline semiconductor film is frequently used in a monolithic liquid crystal electro-optical device in which pixel driving TFTs and driver circuit TFTs are formed on the same glass substrate.
In a laser annealing method that is preferred because of its being mass-producible and industrially superior, a laser beam of a high-power pulse laser such as an excimer laser is used. The laser beam is processed by an optical system so that the subject surface is irradiated with a several centimeters square spot-light like beam or with a linear beam 10 centimeters or more in length while running the laser beam over the subject surface (or moving the laser beam irradiation point relative to the subject surface).
The linear laser beam is particularly highly mass-producible because, unlike spot-light like laser beam which requires both longitudinal and lateral scanning, irradiation of the entire subject surface can be made by running the linear laser beam only in the direction perpendicular to the longitudinal direction of the linear beam. The reason for running the linear beam perpendicular to the longitudinal direction of the linear beam is that it is the most efficient scanning direction. Being thus highly mass-producible, to use the linear laser beam obtained by processing a pulse oscillation excimer laser beam through an appropriate optical system is becoming the mainstream laser annealing method.
FIGS. 1A and 1B show an example of the structure of an optical system for processing a laser beam into a linear shape on the subject surface. The structure shown here is very common to a degree that every beam processing optical system is stemmed from this structure of FIGS. 1A and 1B. According to this structure, energy of the laser beam is homogenized on the subject surface as well as the laser beam is shaped into a linear shape on the subject surface. An optical system that homogenizes the energy of a laser beam is called a beam homogenizer in general.
When an excimer laser that is an ultraviolet ray is used as a light source, it is preferable to form the entire optical system from quartz because this gives a high transmittance. An appropriate coating is one that has a transmittance of 99% or more regarding with the wavelength of an excimer laser used.
Reference is made first to a side view of FIG. 1A. A laser beam emitted from a laser emitter 101 is split in the direction perpendicular to the travel direction of the laser beam by cylindrical array lenses 102a and 102b. The perpendicular direction is herein referred to as longitudinal direction. The longitudinal direction is bent toward the direction of light reflected by a mirror provided in an intermediate point in the optical system. In the illustrated structure, a laser beam is split into four. These split laser beams are once bound together by a cylindrical array lens 104. The beams are then reflected by a mirror 107, and unified back to a single laser beam on an irradiation surface 109 by a doublet cylindrical lens 108. The doublet cylindrical lens is a lens composed of two cylindrical lenses. In this way, the energy of the linear laser beam along its shorter side is homogenized and the length of the shorter side is determined.
Turning next to a top view of FIG. 1B, a laser beam emitted from the laser emitter 101 is split in the direction perpendicular to the travel direction of the laser beam and perpendicular to the longitudinal direction by a cylindrical array lens 103. The direction is herein referred to as lateral direction. The lateral direction is bent toward the direction of light reflected at a mirror provided in an intermediate point in the optical system. According to the illustrated structure, the laser beam is split into seven. Thereafter, the split laser beams are combined into one beam on the subject surface 109 by the cylindrical array lens 104. Thus the energy of the linear beam is homogenized along its longer side and the length thereof is determined.
The lenses mentioned above are made of synthetic quartz in order to process an excimer laser. Also, the surfaces of the lenses are coated so that an excimer laser can easily be transmitted. These configurations give each lens a transmittance of 99% or more regarding with the excimer laser.
By running linear laser beams processed by a system structured as above in a manner that makes one linear beam overlap with its precedent laser beam along their shorter sides, the entire surface of the amorphous semiconductor can be irradiated and annealed with the laser to be crystallized. (And if this laser annealing is performed on a partially crystallized semiconductor, its crystallinity can be improved.)
Now, a typical manufacturing method of a semiconductor film to be an irradiation subject is described. First, a 5 inch square Corning 1737 substrate 0.7 mm in thickness is prepared. Using a plasma CVD device, an SiO2 film (silicon oxide film) with a thickness of 200 nm is formed on the substrate and an amorphous silicon film (hereinafter referred to as a-Si film) with a thickness of 50 nm is formed on the surface of the SiO2 film. Then the substrate is exposed to nitrogen gas at a temperature of 500° C. for an hour to reduce the hydrogen concentration of the film. The film is thus remarkably improved in laser resistivity.
A laser apparatus used here is the XeCl excimer laser L 3308 (wavelength: 308 nm, pulse width: 30 ns) manufactured by Lambda Physik, Inc. This laser apparatus emits a pulse oscillation laser and is capable of outputting an energy of 500 mJ/pulse. The size of the laser beam is 10×30 mm (both are half-width in the beam profile) at its exit. The term laser beam exit is defined herein as a plane perpendicular to the travel direction of a laser beam immediately after the laser beam is emitted from the laser irradiation apparatus.
A laser beam emitted from an excimer laser is generally shaped into a rectangular shape and is within 3 to 5 aspect ratio. As to the intensity of the laser beam, it has a Gaussian distribution with the center of the laser beam being the strongest. The laser beam is changed by the optical system structured as shown in FIGS. 1A and 1B into a 125 mm×0.4 mm linear beam having a uniform energy distribution.
FIGS. 2A to 2D are top views of the substrate while being irradiated with two pulses of the linear beam. The overlap pitch of the linear beam is different among FIGS. 2A to 2D. When the above semiconductor film is irradiated with the laser, the optimal overlap pitch is about 1/10 of the beam width (half-width in the beam profile) of the linear beam as shown in FIG. 2A. This improves uniformity in crystallinity of the film. The half-width in the above example is 0.4 mm, and hence the laser beam irradiation is made by setting the pulse frequency of the excimer laser to 30 Hz and the scanning rate thereof to 1.0 mm/s. The energy density on the surface irradiated with the laser beam is 420 mJ/cm2 at this point. This method described here is a very common one used in crystallizing a semiconductor film with a linear laser beam.
When laser annealing is conducted using an optical system as the one shown in FIGS. 1A and 1B, a laser beam is processed to have a linear shape so that the subject surface is irradiated with a linear beam. As shown in FIG. 2A, the overlap pitch of the linear beam is in the order of 1/10 of the beam width (half-width in the beam profile).
The wavelength of an excimer laser is 308 nm as shown in FIG. 3. Therefore the absorption coefficient at this wavelength is 1.38×106 cm−1 in an amorphous silicon film and 1.56×106 cm−1 in a polycrystalline silicon film. Thus the absorption coefficient of the excimer laser is the same for an amorphous silicon film and a polycrystalline silicon film.
These facts result in that recrystallization of already crystallized portion repeatedly occur in laser annealing with an excimer laser. This is the cause of fluctuation in grain size.
The longer side of the linear beam is about 100 mm in length under the present circumstances. Even when a beam expander is used, the longer side of the linear beam can only be extended to have a length of 150 mm at most without sacrificing the uniformity of the linear beam.
On the other hand, there is the trend toward larger-area substrates and it is not uncommon to use, for example, a 320 mm×400 mm substrate and an 8 inch circular substrate (about 200 mm in diameter). Shown in FIGS. 4A to 4C and in FIGS. 26A to 26C are examples of irradiating such a large-area substrate with the linear laser beam. FIGS. 4A to 4C and FIGS. 26A to 26C each show an example in which a 320 mm ×400 mm substrate is irradiated with a linear laser beam whose longer side is 15 cm in length. In the various irradiation methods illustrated in FIGS. 4A to 4C and FIGS. 26A to 26C, the linear beam overlaps with another linear beam at the center of the substrate (FIG. 4A and FIG. 26A), or the center of the substrate is not irradiated with laser (FIG. 4B and FIG. 26B). Both ends along the longer side of the linear beam are significantly lower in terms of energy density than around the center of the linear beam. Therefore the crystallinity at the center of the substrate is not well also in the case of employing the irradiation methods shown in FIGS. 4C and 26C. It is thus impossible to obtain a desirable electric characteristic in measuring the electric characteristic of a TFT formed from the central portion of the substrate.